Organic nitrite additives for nonaqueous electrolyte in alkali metal electrochemical cells

ABSTRACT

An alkali metal, solid cathode, nonaqueous electrochemical cell capable of delivering high current pulses, rapidly recovering its open circuit voltage and having high current capacity, is described. The stated benefits are realized by the addition of at least one nitrite additive to an electrolyte comprising an alkali metal salt dissolved in a mixture of a low viscosity solvent and a high permittivity solvent. A preferred solvent mixture includes propylene carbonate, dimethoxyethane and an alkyl nitrite additive.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention generally relates to an alkali metalelectrochemical cell, and more particularly, to an alkali metal cellsuitable for current pulse discharge applications with improveddischarge performance. Still more particularly, the present inventionrelates to a lithium electrochemical cell activated with an electrolytehaving an additive for the purpose of providing increased dischargevoltages and reducing and/or eliminating voltage delay under currentpulse discharge applications. Voltage delay is a phenomenon typicallyexhibited in an alkali metal/transition metal oxide cell, andparticularly, a lithium/silver vanadium oxide cell, that has beendepleted of 40% to 70% of its capacity and is subjected to current pulsedischarge applications. According to the present invention, thepreferred additive to the activating electrolyte for such a chemistry isan organic nitrite compound.

The voltage response of a cell which does not exhibit voltage delayduring the application of a short duration pulse or pulse train hasdistinct features. First, the cell potential decreases throughout theapplication of the pulse until it reaches a minimum at the end of thepulse, and second, the minimum potential of the first pulse in a seriesof pulses is higher than the minimum potential of the last pulse.

On the other hand, the voltage response of a cell which exhibits voltagedelay during the application of a short duration pulse or during a pulsetrain can take one or both of two forms. One form is that the leadingedge potential of the first pulse is lower than the end edge potentialof the first pulse. In other words, the voltage of the cell at theinstant the first pulse is applied is lower than the voltage of the cellimmediately before the first pulse is removed. The second form ofvoltage delay is that the minimum potential of the first pulse is lowerthan the minimum potential of the last pulse when a series of pulseshave been applied.

The initial drop in cell potential during the application of a shortduration pulse reflects the resistance of the cell, i.e., the resistancedue to the cathode, the cathode-electrolyte interphase, the anode andthe anode-electrolyte interphase. In the absence of voltage delay, theresistance due to passivated films on the anode and/or cathode isnegligible. However, the formation of a surface film is unavoidable foralkali metal, and in particular, lithium metal anodes, and for lithiumintercalated carbon anodes, due to their relatively low potential andhigh reactivity towards organic electrolytes. Thus, the ideal anodesurface film should be electrically insulating and tonically conducting.While most alkali metal, and in particular, lithium electrochemicalsystems meet the first requirement, the second requirement is difficultto achieve. In the event of voltage delay, the resistance of these filmsis not negligible, and as a result, impedance builds up inside the celldue to this surface layer formation which often results in reduceddischarge voltage and reduced cell capacity. In other words, the drop inpotential between the background voltage and the lowest voltage underpulse discharge conditions, excluding voltage delay, is an indication ofthe conductivity of the cell, i.e., the conductivity of the cathode,anode, electrolyte, and surface films, while the gradual decrease incell potential during the application of the pulse train is due to thepolarization of the electrodes and electrolyte.

Thus, decreased discharge voltages and the existence of voltage delayare undesirable characteristics of alkali metal/mixed metal oxide cellssubjected to current pulse discharge conditions in terms of theirinfluence on devices such as medical devices including implantablepacemakers and cardiac defibrillators. Depressed discharge voltages andvoltage delay are undesirable because they limit the effectiveness andeven the proper functioning of both the cell and the associatedelectrically powered device under current pulse discharge conditions.

2. Prior Art

One of the known solutions to the above problems is to saturate theelectrolyte solution with carbon dioxide CO₂. Cycling efficiency isimproved dramatically in secondary cell systems having a lithium anodeactivated with CO₂ saturated electrolytes (V. R. Koch and S. B. Brummer,Electrochimica Acta, 1978, 23, 55-62; U.S. Pat. No. 4,853,304 to Ebneret al.; D. Aurbach, Y. Gofer, M. Ben-Zion and P. Aped, J. Electroanal.Chem. 1992, 339, 451-471). U.S. Pat. No. 5,569,558 to Takeuchi et al.relates to the provision of a CO₂ saturated electrolyte for alleviatingthe presence of voltage delay in primary cells having a mixed transitionmetal oxide cathode such as lithium/silver vanadium oxide cells. Thesame effect is also known for lithium intercalated carbon anodesecondary batteries (D. Aurbach, Y. Ein-Eli, O. Chusid, Y. Carmeli, M.Babai and H. Yamin, J. Electrochem. Soc. 1994, 141, 603-611). Sulfurdioxide (SO₂) has also been reported to be another additive thatimproves charge-discharge cycling in rechargeable lithium ion cells (Y.Ein-Eli, S. R. Thomas and V. R. Koch, J. Electrochem. Soc. 1996, 143,L195-L197).

In spite of the success of CO₂ and SO₂ in improving cell dischargecharacteristics, their use has been limited. One problem associated withboth CO₂ and SO₂ as electrolyte additives is that they are in a gaseousstate at room temperature, and are thus difficult to handle. Also, it isdifficult to control the dissolved concentration of CO₂. Best resultsare achieved at pressures of up to 50 psig., which further detracts fromthe practicality of this additive.

Instead of carbon dioxide and sulfur dioxide, the present invention isdirected to the provision of organic nitrite additives in theelectrolyte of an alkali metal electrochemical cell to beneficiallymodify the anode surface film. The nitrite additives are defined hereinas organic alkyl nitrite compounds provided as a co-solvent withcommonly used organic aprotic solvents. The organic nitrite additivesare in a condensed phase which makes them easy to handle in electrolytepreparation. When used as a co-solvent in an activating electrolyte, thenitrite additives interact with the alkali metal anode to form anionically conductive surface protective layer thereon. The conductivesurface layer improves the discharge performance of the alkali metalelectrochemical cell by providing increased discharge voltages andminimizing or even eliminating voltage delay in the high current pulsedischarge of such cells.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the pulse dischargeperformance of an alkali metal electrochemical cell, and moreparticularly a primary lithium electrochemical cell, by the provision ofat least one of a family of nitrite additives, preferably an alkylcompound as a co-solvent in the cell's activating nonaqueous electrolytesolution. Due to the high reduction potential of the nitrite group vs.lithium, the nitrite additives compete effectively with the otherelectrolyte co-solvents or the solute to react with the lithium anode.Lithium nitrite or the lithium salt of nitrite reduction products arebelieved to be the major reaction products. These lithium salts arebelieved to deposit on the anode surface to form an ionically conductiveprotective film thereon. As a consequence, the chemical composition andperhaps the morphology of the anode surface protective layer is changed,and this proves beneficial to the discharge characteristics of the cell.

The thusly fabricated cell exhibits higher discharge potentials andimproved voltage delay under current pulse discharge usage, which is anunexpected result. More particularly, the present invention is directedto the introduction of at least one nitrite additive into theelectrolyte of a lithium/silver vanadium oxide electrochemical cell forthe purpose of improving the pulse minimum potentials and reducingand/or eliminating voltage delay during pulse discharging applications.Alkali metal/transition metal oxide electrochemical systems aretypically activated with an electrolyte comprising a relatively lowviscosity solvent and a relatively high permittivity solvent; The soluteof the electrolyte is an inorganic alkali metal salt wherein the alkalimetal of the salt is the same as the alkali metal of the anode. Thenitrite compound of the present invention is introduced into theelectrolyte as an additive to interact with the alkali metal anode, andparticularly with the lithium anode, to form an ionically conductiveprotective anode surface layer which improves the discharge performanceof the cell through higher discharge potentials under current pulsedischarge conditions. Therefore, the present invention is directed to anovel electrolyte solution provided in operative association with anelectrochemical system incorporated into a defibrillator battery toimprove discharge performance under high current pulse dischargeconditions.

These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description and to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 are graphs constructed from the pulse train 4 waveformsof Li/SVO cells activated with a nonaqueous electrolyte devoid of analkyl nitrite compound and having 0.005M and 0.01M t-butyl nitritedissolved therein, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term "pulse" means a short burst of electricalcurrent of a significantly greater amplitude than that of a prepulsecurrent immediately prior to the pulse. A pulse train consists of atleast two pulses of electrical current delivered in relatively shortsuccession with or without open circuit rest between the pulses.

The electrochemical cell of the present invention includes an anodeselected from Groups IA, IIA or IIIB of the Periodic Table of Elements,including lithium, sodium, potassium, etc., and their alloys andintermetallic compounds including, for example Li--Si, Li--B andLi--Si--B alloys and intermetallic compounds. The preferred anodecomprises lithium, and the more preferred anode comprises a lithiumalloy, the preferred lithium alloy being a lithium-aluminum alloy. Thegreater the amount of aluminum present by weight in the alloy, however,the lower the energy density of the cell.

The form of the anode may vary, but preferably the anode is a thin metalsheet or foil of the anode metal, pressed or rolled on a metallic anodecurrent collector, i.e., preferably comprising nickel, to form an anodecomponent. In the exemplary cell of the present invention, the anodecomponent has an extended tab or lead of the same material as the anodecurrent collector, i.e., preferably nickel, integrally formed therewithsuch as by welding and contacted by a weld to a cell case of conductivemetal in a case-negative electrical configuration. Alternatively, theanode may be formed in some other geometry, such as a bobbin shape,cylinder or pellet to allow an alternate low surface cell design.

The cathode is preferably of a solid material and the electrochemicalreaction at the cathode involves conversion of ions which migrate fromthe anode to the cathode in atomic or molecular forms. The solid cathodematerial may comprise a metal, a metal oxide, a mixed metal oxide, ametal sulfide or a carbonaceous compound, and combinations thereof. Themetal oxide, the mixed metal oxide and the metal sulfide can be formedby the chemical addition, reaction, or otherwise intimate contact ofvarious metal oxides, metal sulfides and/or metal elements, preferablyduring thermal treatment, sol-gel formation, chemical vapor depositionor hydrothermal synthesis in mixed states. The active materials therebyproduced contain metals, oxides and sulfides of Groups IB, IIB, IIIB,IVB, VB, VIB, VIIB and VIII, which includes the noble metals and/orother oxide and sulfide compounds.

One preferred mixed metal oxide has the general formula SM_(x) V₂ O_(y)wherein SM is a metal selected from Groups IB to VIIB and VIII of thePeriodic Table of Elements, wherein x is about 0.30 to 2.0 and y isabout 4.5 to 6.0 in the general formula. By way of illustration, and inno way intended to be limiting, one exemplary cathode active materialcomprises silver vanadium oxide (SVO) having the general formula Ag_(x)V₂ O_(y) in any one of its many phases, i.e., β-phase silver vanadiumoxide having in the general formula x=0.35 and y=5.8, γ-phase silvervanadium oxide having in the general formula x=0.74 and y=5.37 andε-phase silver vanadium oxide having in the general formula x=1.0 andy=5.5, and combination and mixtures of phases thereof. For a moredetailed description of such a cathode active material, reference ismade to U.S. Pat. No. 4,310,609 to Liang et al., which is assigned tothe assignee of the present invention and incorporated herein byreference.

Another preferred composite cathode active material includes V₂ O_(z)wherein z≦5 combined with Ag₂ O with the silver in either thesilver(II), silver(I) or silver(0) oxidation state and CuO with thecopper in either the copper(II), copper(I) or copper(0) oxidation stateto provide the mixed metal oxide having the general formula Cu_(x)Ag_(y) V₂ O_(z), (CSVO). Thus, this composite cathode active materialmay be described as a metal oxide-metal oxide-metal oxide, a metal-metaloxide-metal oxide, or a metal-metal-metal oxide and the range ofmaterial compositions found for Cu.sub. Ag_(y) V₂ O_(z) is preferablyabout 0.01≦x≦1.0, about 0.01 ≦Y≦1.0 and about 5.01≦z≦6.5. Typical formsof CSVO are Cu₀.16 Ag₀.67 V₂ O_(z) with z being about 5.5 and Cu₀.5Ag₀.5 V₂ O_(z) with z being about 5.75. The oxygen content is designatedby z since the exact stoichiometric proportion of oxygen in CSVO canvary depending on whether the cathode material is prepared in anoxidizing atmosphere such as air or oxygen, or in an inert atmospheresuch as argon, nitrogen and helium. For a more detailed description ofthis cathode active material, reference is made to U.S. Pat. Nos.5,472,810 to Takeuchi et al. and 5,516,340 to Takeuchi et al., both ofwhich are assigned to the assignee of the present invention andincorporated herein by reference.

Additional cathode active materials include manganese dioxide, lithiumcobalt oxide, lithium nickel oxide, copper vanadium oxide, titaniumdisulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide,and fluorinated carbon, and mixtures thereof. Preferably, the cathodecomprises from about 80 to about 99 weight percent of the cathode activematerial.

Cathode active materials prepared as described above are preferablymixed with a binder material such as a powdered fluoro-polymer, morepreferably powdered polytetrafluoroethylene or powdered polyvinylidenefluoride present at about 1 to about 5 weight percent of the cathodemixture. Further, up to about 10 weight percent of a conductive diluentis preferably added to the cathode mixture to improve conductivity.Suitable materials for this purpose include acetylene black, carbonblack and/or graphite or a metallic powder such as powdered nickel,aluminum, titanium and stainless steel. The preferred cathode activemixture thus includes a powdered fluoro-polymer binder present at about3 weight percent, a conductive diluent present at about 3 weight percentand about 94 weight percent of the cathode active material. The cathodeactive mixture may be in the form of one or more plates operativelyassociated with at least one or more plates of anode material, or in theform of a strip wound with a corresponding strip of anode material in astructure similar to a "jellyroll".

In order to prevent internal short circuit conditions, the cathode isseparated from the Group IA, IIA or IIIB anode material by a suitableseparator material. The separator is of electrically insulativematerial, and the separator material also is chemically unreactive withthe anode and cathode active materials and both chemically unreactivewith and insoluble in the electrolyte. In addition, the separatormaterial has a degree of porosity sufficient to allow flow therethroughof the electrolyte during the electrochemical reaction of the cell.Illustrative separator materials include woven and non-woven fabrics ofpolyolefinic fibers or fluoropolymeric fibers including polyvinylidenefluoride, polyethylenetetrafluoroethylene, andpolyethylenechlorotrifluoroethylene laminated or superposed with apolyolefinic or a fluoropolymeric microporous film. Suitable microporousfilms include a polytetrafluoroethylene membrane commercially availableunder the designation ZITEX (Chemplast Inc.), polypropylene membranecommercially available under the designation CELGARD (Celanese PlasticCompany, Inc.) and a membrane commercially available under thedesignation DEXIGLAS (C.H. Dexter, Div., Dexter Corp.). The separatormay also be composed of non-woven glass, glass fiber materials andceramic materials.

The form of the separator typically is a sheet which is placed betweenthe anode and cathode electrodes and in a manner preventing physicalcontact therebetween. Such is the case when the anode is folded in aserpentine-like structure with a plurality of cathode plates disposedintermediate the anode folds and received in a cell casing or when theelectrode combination is rolled or otherwise formed into a cylindrical"jellyroll" configuration.

The electrochemical cell of the present invention further includes anonaqueous, ionically conductive electrolyte operatively associated withthe anode and the cathode electrodes. The electrolyte serves as a mediumfor migration of ions between the anode and the cathode during theelectrochemical reactions of the cell and nonaqueous solvents suitablefor the present invention are chosen so as to exhibit those physicalproperties necessary for ionic transport (low viscosity, low surfacetension and wettability). Suitable nonaqueous solvents are comprised ofan inorganic salt dissolved in a nonaqueous solvent and more preferablyan alkali metal salt dissolved in a mixture of aprotic organic solventscomprising a low viscosity solvent including organic esters, ethers anddialkyl carbonates, and mixtures thereof, and a high permittivitysolvent including cyclic carbonates, cyclic esters and cyclic amides,and mixtures thereof. Low viscosity solvents include tetrahydrofuran(THF), methyl acetate (MA), diglyme, triglyme, tetraglyme,1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1-ethoxy,2-methoxyethane (EME), dimethyl carbonate (DMC), diethyl carbonate(DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methylpropyl carbonate (MPC) and ethyl propyl carbonate (EPC), and mixturesthereof. High permittivity solvents include propylene carbonate (PC),ethylene carbonate (EC), butylene carbonate (BC), acetonitrile, dimethylsulfoxide, dimethyl formamide, dimethyl acetamide, γ-butyrolactone (GBL)and N-methyl-pyrrolidinone (NMP), and mixtures thereof.

The preferred electrolyte comprises an inorganic alkali metal salt, andin the case of an anode comprising lithium, the alkali metal salt of theelectrolyte is a lithium based salt. Known lithium salts that are usefulas a vehicle for transport of alkali metal ions from the anode to thecathode include LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO4, LiAlCl₄, LiGaCl₄,LiC(SO₂ CF₃)₃, LiN(SO₂ CF₃)₂, LiSCN, LiO₃ SCF₂ CF₃, LiC₆ F₅ SO₃, LiO₂CCF₃, LiSO₃ F, LiB(C₆ H₅)₄ and LiCF₃ SO₃, and mixtures thereof. Suitablesalt concentrations typically range between about 0.8 to 1.5 molar, anda preferred electrolyte for a lithium/transition metal oxideelectrochemical cell includes LiAsF₆ or LiPF₆ dissolved in a 50:50mixture, by volume, of PC and DME.

In accordance with the present invention, at least one organic nitriteadditive is provided-as a co-solvent in the electrolyte solution of thepreviously described alkali metal electrochemical cell. The nitriteadditive is preferably an alkyl nitrite compound having the generalformula (RO)N(═O), wherein R is an organic group of either a saturatedor unsaturated hydrocarbon or heteroatom substituted saturated orunsaturated organic group containing 1 to 10 carbon atoms. The greatesteffect is found when methyl nitrite, ethyl nitrite, propyl nitrite,isopropyl nitrite, butyl nitrite, isobutyl nitrite, t-butyl nitrite,benzyl nitrite and phenyl nitrite, and mixtures thereof are used asadditives in the electrolyte.

The above described compounds are only intended to be exemplary of thosethat are useful with the present invention, and are not to be construedas limiting. Those skilled in the art will readily recognize nitritecompounds which come under the purview of the general formula set forthabove and which will be useful as additives for the electrolyte toreduce voltage delay according to the present invention.

While not intending to be bound by any particular mechanism, it isbelieved that due to the presence of the N═O bond in the nitritefunctional group, [--O--N(═O)], the bond between oxygen and the R groupis severed and the nitrite intermediate is able to compete effectivelywith the other electrolyte solvents or solutes to react with lithium andform a nitrite salt, i.e., lithium nitrite, or the lithium salt of anitrite reduction product on the surface of the anode. The resultingsalt is tonically more conductive than lithium oxide which may form onthe anode in the absence of the organic nitrite additive. As aconsequence, the chemical composition and perhaps the morphology of theanode surface protective layer is believed to be changed withconcomitant benefits to the cell's discharge characteristics.

In the present invention, the anode is lithium metal and the cathode ispreferably the transition mixed metal oxide AgV₂ O₅.5 (SVO). Thepreferred electrolyte is 1.0M to 1.2M LiAsF₆ dissolved in an aproticsolvent mixture comprising at least one of the above listed lowviscosity solvents and at least one of the above listed highpermittivity solvents. The preferred aprotic solvent mixture comprises a50/50, by volume, mixture of propylene carbonate and dimethoxyethane.The concentration of the above discussed nitrite additives according tothe present invention should preferably be in the range of between about0.005M to about 0.01M. The positive effects of these additives inincreasing discharge voltages and reducing voltage delay in a pulsedischarging alkali metal cell have been achieved both at roomtemperature as well as at temperatures up to about 37° C. This makes thenovel electrolyte solution of the present invention particularly usefulfor activating an alkali metal/transition metal oxide cell incorporatedinto an implantable medical device such as a cardiac defibrillator tominimize or even eliminate voltage delay and provide improved minimumpotentials under high current pulse discharge conditions.

As is well known by those skilled in the art, an implantable cardiacdefibrillator is a device that requires a power source for a generallymedium rate, constant resistance load component provided by circuitsperforming such functions as, for example, the heart sensing and pacingfunctions. From time to time, the cardiac defibrillator may require agenerally high rate, pulse discharge load component that occurs, forexample, during charging of a capacitor in the defibrillator for thepurpose of delivering an electrical shock to the heart to treattachyarrhythmias, the irregular, rapid heart beats that can be fatal ifleft uncorrected. Increased pulse minimum potentials and reduction, andeven elimination of voltage delay during a current pulse application isimportant for proper device operation and extended device life.

The assembly of the cell described herein is preferably in the form of awound element cell. That is, the fabricated cathode, anode and separatorare wound together in a "jellyroll" type configuration or "wound elementcell stack" such that the anode is on the outside of the roll to makeelectrical contact with the cell case in a case-negative configuration.Using suitable top and bottom insulators, the wound cell stack isinserted into a metallic case of a suitable size dimension. The metalliccase may comprise materials such as stainless steel, mild steel,nickel-plated mild steel, titanium, tantalum or aluminum, but notlimited thereto, so long as the metallic material is compatible for usewith components of the cell.

The cell header comprises a metallic disc-shaped body with a first holeto accommodate a glass-to-metal seal/terminal pin feedthrough and asecond hole for electrolyte filling. The glass used is of a corrosionresistant type having up to about 50% by weight silicon such as CABAL12, TA 23 or FUSITE 425 or FUSITE 435. The positive terminal pinfeedthrough preferably comprises titanium although molybdenum, aluminum,nickel alloy, or stainless steel can also be used. The cell headercomprises elements having compatibility with the other components of theelectrochemical cell and is resistant to corrosion. The cathode lead iswelded to the positive terminal pin in the glass-to-metal seal and theheader is welded to the case containing the electrode stack. The cell isthereafter filled with the electrolyte solution comprising at least oneof the nitrite additives described hereinabove and hermetically sealedsuch as by close-welding a stainless steel ball over the fill hole, butnot limited thereto.

The above assembly describes a case-negative cell, which is thepreferred construction of the exemplary cell of the present invention.As is well known to those skilled in the art, the exemplaryelectrochemical system of the present invention can also be constructedin a case-positive configuration.

The following example describes the manner and process of anelectrochemical cell according to the present invention, and set forththe best mode contemplated by the inventors of carrying out theinvention, but is not construed as limiting.

EXAMPLE I

Lithium anode material was pressed on nickel current collector screenand silver vanadium oxide cathode material was pressed on titaniumcurrent collector screen. A prismatic cell stack assembly configurationwith two layers of microporous membrane polypropylene separatorsandwiched between the anode and cathode was prepared. The electrodeassembly was then hermetically sealed in a stainless steel casing in acase-negative configuration. Three control cells were activated with thestandard electrolyte consisting of 1.0M LiAsF₆ dissolved in a 50:50, byvolume, mixture of PC and DME without an organic nitrite additive (Group1). Fifteen cells (three cells per group were activated with the sameelectrolyte used to activate the Group 1 cells and further containing0.005M, 0.01M, 0.05M, 0.10M, or 0.20M t-butyl nitrite (TBN), as setforth in Table 1.

                  TABLE 1                                                         ______________________________________                                        Cell Construction                                                             Group   [LiAsF.sub.6 ]                                                                              PC:DME   [TBN]                                          ______________________________________                                        1       1.0M          50:50    0.00M                                          2       1.0M          50:50     0.005M                                        3       1.0M          50:50    0.01M                                          4       1.0M          50:50    0.05M                                          5       1.0M          50:50    0.10M                                          6       1.0M          50:50    0.20M                                          ______________________________________                                    

A constant resistance load of 3.57 kΩ was applied to all of the cellsfor 21 hours during an initial predischarge period. The predischargeperiod is referred to as burn-in and depleted the cells of approximately1% of their theoretical capacity. following burn-in, the cells weresubjected to acceptance pulse testing consisting of four 10 secondpulses (23.2 mA/cm²) with a 15 second rest between each pulse. Theaverage discharge readings for the pre-pulse potentials, voltage delayand pulse minimum potentials during acceptance pulse testing for thesepulse rains are summarized in Table 2. Voltage delay is calculated aspulse 1 end potential minus pulse 1 minimum potential.

                  TABLE 2                                                         ______________________________________                                        Acceptance Pulse Train Voltages (average)                                     Group [TBN]    Ppre1  V-Delay                                                                              P1min  P1end P4min                               ______________________________________                                        1     0.00M    3.276  0.302  2.261  2.563 2.524                               2      0.005M  3.274  0.239  2.362  2.601 2.540                               3     0.01M    3.274  0.289  2.320  2.609 2.550                               4     0.05M    3.264  0.564  2.042  2.606 2.544                               5     0.10M    3.263  0.673  1.919  2.592 2.533                               6     0.20M    3.264  0.619  1.948  2.567 2.516                               ______________________________________                                    

In the acceptance pulse train, the groups 1 to 3 cells exhibited similarpre-pulse potentials while the groups 4 to 6 cells presented slightlylower pre-pulse potentials. In addition, all of the cells exhibitedvoltage delay. Specifically, under higher TBN concentrations (≧0.05M),the groups 4 to 6 cells presented larger voltage delay and thus lowerP1min potentials than that of the group 1 control cells. The groups 2and 3 cells with relatively low TBN concentrations (≦0.01M) resulted insmaller voltage delay and higher P1min potentials than that of the group1 control cells. Generally, the P1 voltage rapidly recovered for allgroups of cells having the TBN additive, as evidenced by P1 end beingequal or higher than that of the control cells. Except for the group 6cells, all other additive cells showed higher P4min potentials than thatof the control cells.

Following acceptance pulse testing, all of the cells were dischargedunder loads of 9.53 kohms with superimposed pulse trains applied every39 days. The pulse trains consisted of four 10 second pulses (23.2mA/cm²) with 15 seconds rest between each pulse. The average dischargereadings for the pre-pulse potentials, voltage delay and pulse minimumpotentials for pulse trains 1 to 6 are summarized in Tables 3 to 8,respectively.

                  TABLE 3                                                         ______________________________________                                        Pulse Train 1 Voltages (average)                                              Group [TBN]    Ppre1  V-Delay                                                                              P1min  P1end P4min                               ______________________________________                                        1     0.00M    3.224  0.001  2.639  2.640 2.529                               2      0.005M  3.224  0.000  2.647  2.647 2.537                               3     0.01M    3.223  0.000  2.657  2.657 2.548                               4     0.05M    3.221  0.000  2.648  2.648 2.536                               5     0.10M    3.225  0.000  2.638  2.638 2.524                               6     0.20M    3.229  0.000  2.622  2.622 2.507                               ______________________________________                                    

In pulse train 1 (Table 3), all of the cells had similar pre-pulsepotentials and none of them exhibited any voltage delay. Except for thegroup 6 cells, all of the other groups of cells with TBN additive(groups 2 to 5) presented similar or higher pulse minimum potentialsthan that of the group 1 control cells.

                  TABLE 4                                                         ______________________________________                                        Pulse Train 2 Voltages (average)                                              Group [TBN]    Ppre1  V-Delay                                                                              P1min  P1end P4min                               ______________________________________                                        1     0.00M    3.164  0.000  2.548  2.548 2.432                               2      0.005M  3.163  0.012  2.540  2.552 2.437                               3     0.01M    3.163  0.033  2.538  2.571 2.452                               4     0.05M    3.170  0.027  2.550  2.577 2.447                               5     0.10M    3.166  0.019  2.542  2.561 2.436                               6     0.20M    3.163  0.049  2.489  2.538 2.411                               ______________________________________                                    

In pulse train 2 (Table 4), all of the cells with the TBN additive(groups 2 to 6) exhibited relatively small voltage delay (<50 mV), whilethe group 1 control cells did not exhibit any voltage delay. However,the groups 2 to 5 cells presented similar or better P1min, P1 end andP4min potentials in comparison that of the group 1 control cells.

                  TABLE 5                                                         ______________________________________                                        Pulse Train 3 Voltage (average)                                               Group [TBN]    Ppre1  V-Delay                                                                              P1min  P1end P4min                               ______________________________________                                        1     0.00M    2.876  0.000  2.337  2.337 2.258                               2      0.005M  2.876  0.000  2.348  2.348 2.259                               3     0.01M    2.875  0.022  2.339  2.361 2.275                               4     0.05M    2.875  0.255  1.428  1.683 1.962                               5     0.10M    2.762  0.000  1.818  1.818 1.780                               6     0.20M    2.858  0.000  2.281  2.281 1.918                               ______________________________________                                    

In pulse train 3 (Table 5), the groups 5 and 6 cells with the higher TBNconcentrations presented lower pre-pulse potentials than that of groups1 to 4 cells. The groups 1,2,5 and 6 cells did not exhibit any voltagedelay, the group 3 cells exhibited small voltage delay and the group 4cells exhibited large voltage delay. The group 4 to 6 cells with higherTBN concentrations (≧0.05M) presented significantly lower pulse minimumpotentials than that of groups 1 to 3 cells. In contrast, the groups 2and 3 cells with lower TBN concentrations (≦0.01M) still presentedsimilar or higher pulse minimum potentials than that of group 1 controlcells.

                  TABLE 6                                                         ______________________________________                                        Pulse Train 4 Voltages (average)                                              Group [TBN]    Ppre1  V-Delay                                                                              P1min  P1end P4min                               ______________________________________                                        1     0.00M    2.585  0.170  2.026  2.196 2.188                               2      0.005M  2.587  0.062  2.123  2.195 2.183                               3     0.01M    2.583  0.037  2.136  2.173 2.182                               4     0.05M    2.577  0.136  1.623  1.759 1.851                               5     0.10M    2.546  0.177  1.680  1.857 1.840                               6     0.20M    2.553  0.000  1.798  1.798 1.674                               ______________________________________                                    

In pulse train 4 (Table 6), the pre-pulse potentials of all the cellshad reached the 2.6 V voltage plateau. As previously discussed, at thisdepth of discharge SVO cells began to exhibit voltage delaycharacteristics. Accordingly, all of the cells, except the group 6cells, exhibited voltage delay in pulse train 4. In a similar manner asobserved in pulse train 3, the groups 4 to 6 cells presentedsignificantly lower pulse minimum potentials than that of the group 1control cells. However, the groups 2 and 3 cells presented similar P1endand P4min potentials and significantly higher P1min potentials relativeto that of the group 1 control cells.

FIGS. 1 to 3 are presented for illustrative purposes to show thecontrast in pulse train 4 between the group 1 cells without TBNadditive, and the groups 2 and 3 cells which exhibited increased pulseminimum potentials in comparison to the control cells. In particular,curve 10 in FIG. 1 was constructed from the pulse train 4 waveform of arepresentative group 1 cell devoid of the TBN additive while curve 20 inFIG. 2 was constructed from the pulse train 4 waveform of arepresentative group 2 cell activated with the electrolyte having theTBN additive at a concentration of 0.005M and curve 30 in FIG. 3 wasconstructed from the pulse train 4 waveform of a representative group 3cell activated with the nonaqueous electrolyte having the TBN additiveat a concentration of 0.01M.

                  TABLE 7                                                         ______________________________________                                        Pulse Train 5 Voltages (average)                                              Group [TBN]    Ppre1  V-Delay                                                                              P1min  P1end P4min                               ______________________________________                                        1     0.00M    2.538  0.109  1.740  1.849 1.919                               2      0.005M  2.540  0.185  1.690  1.875 1.926                               3     0.01M    2.537  0.199  1.694  1.893 1.935                               4     0.05M    2.538  0.066  1.692  1.758 1.781                               5     0.10M    2.453  0.016  1.499  1.515 1.072                               6     0.20M    2.517  0.054  1.588  1.642 1.425                               ______________________________________                                    

In pulse train 5, all cells exhibited voltage delay. As observed inprevious pulse trains, groups 4 to 6 cells presented significantly lowerP1 end and P4min potentials than that of the groups 1 to 3 cells,although they exhibited smaller voltage delay than that of the latter.Due to the presence of larger voltage delay, the group 2 and 3 cellspresented lower P1min potentials than that of the group 1 control cells.However, the groups 2 and 3 cells still presented higher P1end and P4minpotentials than that of the group 1 control cells.

                  TABLE 8                                                         ______________________________________                                        Pulse Train 6 Voltages (average)                                              Group [TBN]    Ppre1  V-Delay                                                                              P1min  P1end P4min                               ______________________________________                                        1     0.00M    2.517  0.014  1.818  1.832 1.816                               2      0.005M  2.514  0.034  1.790  1.824 1.830                               3     0.01M    2.509  0.088  1.718  1.806 1.810                               4     0.05M    2.497  0.000  1.739  1.739 1.637                               5     0.10M    2.281  0.000  0.737  0.737 0.325                               6     0.20M    2.297  0.000  1.094  1.094 0.409                               ______________________________________                                    

In pulse train 6, only the groups 1 to 3 cells exhibited voltage delay,while the groups 4 to 6 cells with lower prepulse potentials did notexhibit any voltage delay. While the group 1 control cells presentedslightly higher P1min and P1end potentials than that of the groups 2 and3 cells, the latter exhibited similar or higher P4min potentials thanthe former.

Tables 9 and 10 summarize the pulse train data comparison of groups 1 to3 cells with group 1 control cells.

                  TABLE 9                                                         ______________________________________                                        P1end (group X) - P1end (group 1), mV                                               Accep-  Pulse                                                                 tance   Train  Pulse Pulse Pulse Pulse Pulse                            Group Pulse   1      Train 2                                                                             Train 3                                                                             Train 4                                                                             Train 5                                                                             Train 6                          ______________________________________                                        1      0      0      0      0     0     0     0                               2     38      7      4     11     -1   26     -8                              3     46      17     23    24    -23   44    -26                              ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        P4min (group X) - P4min (group 1), mV                                               Accep-  Pulse                                                                 tance   Train  Pulse Pulse Pulse Pulse Pulse                            Group Pulse   1      Train 2                                                                             Train 3                                                                             Train 4                                                                             Train 5                                                                             Train 6                          ______________________________________                                        1      0      0      0     0      0    0      0                               2     16      8      5     1     -5    7     14                               3     26      19     30    17    -6    16    -6                               ______________________________________                                    

As shown in Tables 9 and 10, the groups 2 and 3 cells with TBN additivepresented higher P1end and P4min potentials than that of the group 1control cells in the acceptance pulse train and pulse trains 1, 2, 3 and5. The group 2 cells also had higher P4min potentials in pulse train 6than that of the group 1 control cells. The group 1 cells only presentedhigher P1end and P4min than that of the groups 2 and 3 cells in pulsetrains 4 and 6 except the P4min potentials in pulse train 6 for thegroup 2 cells. Thus, SVO cells with TBN additive at a concentration≦0.01M provide overall improvement for long term pulse performance.

The data in Tables 3 to 10 demonstrate the beneficial effect that TBNhas on increased discharge voltages and improved voltage delay in apulse discharging electrochemical cell. More generally, however, anitrite additive according to the present invention should contain anactivated R--O bond. While not intended to be bound by any particulartheory, it is believed that the formation of (O═)N--(O--Li)_(n) (n=1 or2) or its reaction products deposited on the anode surface isresponsible for the improved performance of an alkali metal/transitionmetal oxide cell, and in particular Li/SVO cells. If the R group in thenitrite additive is activated (t-butyl for example), the O--R bond isrelatively weak. During reduction, the O--R bond breaks to form aproduct containing the N--O--Li salt group. This is believed to be thereason for the observed improvements in the pulse discharge performanceof Li/SVO cells, as exemplified by those having the TBN additive inExample I.

The concentration limit for the TBN additive is preferably about 0.005Mto about 0.01M. Cells with the TBN additive in that concentration rangegenerally presented similar or higher pulse minimum potentials than thatof the control cells throughout pulse trains 1 to 6. The beneficialeffect of that nitrite additive will not be apparent if theconcentration is less than about 0.001M. On the other hand, if the TBNadditive is at a greater concentration of about 0.05M to 0.20M, thebeneficial effect are cancelled by the detrimental effect of higherinternal cell resistance due to the thicker anode surface film formationand lower electrolyte conductivity. This detrimental effect manifests aslower pre-pulse potentials and lower pulse minimum potentials.

Other ones of the listed nitrite additives may have differentconcentration ranges in which their benefits will be apparent. Ingeneral, the beneficial effects of an alkyl nitrite addition in anonaqueous electrolyte according to the present invention will beapparent at concentrations of 0.001M to 0.20M.

Thus, the existence of voltage delay is due to the formation of an anodesurface passivation layer that is ionically less conductive than eitherthe anode material itself or the electrolyte solution. In the presenceof an alkyl nitrite additive according to the present invention at aconcentration of about 0.005M to 0.20M, the anode passivation layer ischemically modified to be ionically more conductive than the passivationlayer formed without the benefit of the additive. It is believed thatdue to the presence of the [--O--N(═O)] functional group, the reductivecleavage of the RO bond in the nitrite additives of the presentinvention may produce lithium nitrite or the lithium salt of a nitritereduction product on the anode surface. This surface film is ionicallymore conductive than the film formed in the absence of the additives andit is responsible for the increased cell performance, especially duringpulse discharge applications. As a consequence, higher pulse minimumpotentials result when an alkali metal/transition metal oxide coupleactivated with a nonaqueous organic solvent having a nitrite additivedissolved therein according to the present invention is subjected to apulse discharge application. This is particularly important inimplantable medical devices powered by a cell according to the presentinvention.

It is appreciated that various modifications to the inventive conceptsdescribed herein may be apparent to those of ordinary skill in the artwithout departing from the spirit and scope of the present invention asdefined by the appended claims.

What is claimed is:
 1. An electrochemical cell, which comprises:a) ananode comprising an alkali metal; b) a solid cathode of electricallyconductive material; and c) a nonaqueous electrolyte activating theanode and the cathode, the nonaqueous electrolyte comprising:i) anitrite additive having the formula: (RO)N(═O), wherein R is an organicgroup of either a saturated hydrocarbon or heteroatom group containing 1to 10 carbon atoms or an unsaturated hydrocarbon or heteroatom groupcontaining 2 to 10 carbon atoms; and ii) an alkali metal salt dissolvedtherein.
 2. The electrochemical cell of claim 1 wherein the nitriteadditive is selected from the group consisting of methyl nitrite, ethylnitrite, propyl nitrite, isopropyl nitrite, butyl nitrite, isobutylnitrite, t-butyl nitrite, benzyl nitrite and phenyl nitrite, andmixtures thereof.
 3. The electrochemical cell of claim 1 wherein thenitrite additive is present in the electrolyte in a range of about0.001M to about 0.20M.
 4. The electrochemical cell of claim 1 whereinthe nitrite additive is t-butyl nitrite present in the electrolyte at aconcentration up to about 0.01M.
 5. The electrochemical cell of claim 1wherein the activated anode and cathode provide the electrochemical celldischargeable to deliver at least one current pulse of an electricalcurrent of a greater amplitude than that of a prepulse currentimmediately prior to the pulse.
 6. The electrochemical cell of claim 5wherein there are at least two pulses delivered in succession with orwithout an open circuit period between the pulses.
 7. Theelectrochemical cell of claim 5 wherein the current pulse is of about23.2 mA/cm².
 8. The electrochemical cell of claim 1 wherein theelectrolyte has a first solvent selected from the group consisting of anester, an ether and a dialkyl carbonate, and mixtures thereof.
 9. Theelectrochemical cell of claim 8 wherein the first solvent is selectedfrom the group consisting of tetrahydrofuran, methyl acetate, diglyme,triglyme, tetraglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane,1-ethoxy,2-methoxyethane, dimethyl carbonate, diethyl carbonate,dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate andethyl propyl carbonate, and mixtures thereof.
 10. The electrochemicalcell of claim 1 wherein the electrolyte has a second solvent selectedfrom the group consisting of a cyclic carbonate, a cyclic ester and acyclic amide, and mixtures thereof.
 11. The electrochemical cell ofclaim 10 wherein the second solvent is selected from the groupconsisting of propylene carbonate, ethylene carbonate, butylenecarbonate, acetonitrile, dimethyl sulfoxide, dimethyl formamide,dimethyl acetamide, γ-butyrolactone and N-methyl-pyrrolidinone, andmixtures thereof.
 12. The electrochemical cell of claim 1 wherein thealkali metal salt is selected from the group consisting of LiPF₆, LiBF₄,LiAsF₆, LiSbF₆, LiClO4, LiAlCl₄, LiGaCl₄, LiC(SO₂ CF₃)₃, LiN(SO₂ CF₃)₂,LiSCN, LiO₃ SCF₂ CF₃, LiC₆ F₅ SO₃, LiO₂ CCF₃, LiSO₃ F, LiB(C₆ H₅)₄ andLiCF₃ SO₃, and mixtures thereof.
 13. The electrochemical cell of claim 1wherein the solid cathode is selected from the group consisting ofsilver vanadium oxide, copper silver vanadium oxide, manganese dioxide,cobalt oxide, nickel oxide, fluorinated carbon, copper oxide, coppersulfide, iron sulfide, iron disulfide, titanium disulfide and coppervanadium oxide, and mixtures thereof.
 14. The electrochemical cell ofclaim 1 wherein the anode is comprised of lithium or a lithium-aluminumalloy.
 15. The electrochemical cell of claim 1 wherein the cathodecomprises from about 80 to about 99 weight percent of the cathode activematerial.
 16. The electrochemical cell of claim 1 wherein the cathodefurther comprises a binder material and a conductive additive.
 17. Theelectrochemical cell of claim 16 wherein the binder material is afluoro-resin powder.
 18. The electrochemical cell of claim 16 whereinthe conductive additive is selected from the group consisting of carbon,graphite powder and acetylene black and metallic powder selected fromthe group consisting of titanium, aluminum, nickel and stainless steel,and mixtures thereof.
 19. The electrochemical cell of claim 1 whereinthe cathode comprises from about 0 to 3 weight percent carbon, about 1to 5 weight percent of a powder fluoro-resin and about 94 weight percentof the cathode active material.
 20. The electrochemical cell of claim 1associated with an implantable medical device powered by the cell. 21.In combination with an implantable medical device requiring at least onecurrent pulse for a medical device operating function, anelectrochemical cell which is dischargeable to deliver the current pulsewhile exhibiting reduced voltage delay, the cell which comprises:a) ananode comprising an alkali metal; b) a solid cathode of electricallyconductive material; and c) a nonaqueous electrolyte activating theanode and the cathode, the nonaqueous electrolyte comprising:i) a firstsolvent selected from the group consisting of an ester, an ether and adialkyl carbonate, and mixtures thereof; ii) a second solvent selectedfrom the group consisting of a cyclic carbonate, a cyclic ester and acyclic amide, and mixtures thereof; iii) a nitrite additive having theformula: (RO)N(═O), wherein R is an organic group of either a saturatedhydrocarbon or heteroatom group containing 1 to 10 carbon atoms or anunsaturated hydrocarbon or heteroatom group containing 2 to 10 carbonatoms; and iv) an alkali metal salt dissolved therein, wherein thealkali metal of the salt is similar to the alkali metal comprising theanode, and wherein the activated anode and cathode provide theelectrochemical cell dischargeable to deliver at least one current pulsefor the medical device operating function, wherein the current pulse isof an electrical current of a greater amplitude than that of a prepulsecurrent immediately prior to the pulse.
 22. The combination of claim 21wherein the nitrite additive is selected from the group consisting ofmethyl nitrite, ethyl nitrite, propyl nitrite, isopropyl nitrite, butylnitrite, isobutyl nitrite, t-butyl nitrite, benzyl nitrite and phenylnitrite, and mixtures thereof.
 23. The combination of claim 21 whereinthe nitrite additive is present in the electrolyte in a range of about0.001M to about 0.20M.
 24. The combination of claim 21 wherein there areat least two pulses delivered in succession with or without an opencircuit period between the pulses.
 25. A method for providing anelectrochemical cell activated with a nonaqueous electrolyte, comprisingthe steps of:a) providing an anode comprising an alkali metal; b)providing a solid cathode of electrically conductive material; and c)activating the electrochemical cell with the nonaqueous electrolyteoperatively associated with the anode and the cathode, the nonaqueouselectrolyte comprising:i) a first solvent selected from the groupconsisting of an ester, an ether and a dialkyl carbonate, and mixturesthereof; ii) a second solvent selected from the group consisting of acyclic carbonate, a cyclic ester and a cyclic amide, and mixturesthereof; iii) a nitrite additive having the formula: (RO)N(═O), whereinR is an organic group of either a saturated hydrocarbon and heteroatomgroup containing 1 to 10 carbon atoms or an unsaturated hydrocarbon orheteroatom group containing 2 to 10 carbon atoms; and iv) an alkalimetal salt dissolved therein, wherein the alkali metal of the salt issimilar to the alkali metal comprising the anode.
 26. The method ofclaim 25 including selecting the nitrite additive from the groupconsisting of methyl nitrite, ethyl nitrite, propyl nitrite, isopropylnitrite, butyl nitrite, isobutyl nitrite, t-butyl nitrite, benzylnitrite and phenyl nitrite, and mixtures thereof.
 27. The method ofclaim 25 wherein the nitrite additive is present in the electrolyte in arange of about 0.001M to about 0.20M.
 28. The method of claim 25 whereinthe nitrite additive is t-butyl nitrite present in the electrolyte at aconcentration up to about 0.01M.
 29. The method of claim 25 includingdischarging the cell to deliver at least one current pulse of anelectrical current of a greater amplitude than that of a prepulsecurrent immediately prior to the pulse.
 30. The method of claim 29including discharging the cell to deliver at least two current pulses insuccession with or without an open circuit period between the pulses.31. The method of claim 29 wherein the current pulse is of about 23.2mA/cm².
 32. The method of claim 25 including selecting the first solventfrom the group consisting of tetrahydrofuran, methyl acetate, diglyme,triglyme, tetraglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy,2-methoxyethane, dimethyl carbonate, diethyl carbonate, dipropylcarbonate, ethyl methyl carbonate, methyl propyl carbonate and ethylpropyl carbonate, and mixtures thereof.
 33. The method of claim 25including selecting the second solvent from the group consisting ofpropylene carbonate, ethylene carbonate, butylene carbonate,acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethylacetamide, γ-butyrolactone and N-methyl-pyrrolidinone, and mixturesthereof.
 34. The method of claim 25 including selecting the alkali metalsalt from the group consisting of LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄,LiAlCl₄, LiGaCl₄, LiC(SO₂ CF₃)₃, LiN(SO₂ CF₃)₂, LiSCN, LiO₃ SCF₂ CF₃,LiC₆ F₅ SO₃, LiO₂ CCF₃, LiSO₃ F, LiB(C₆ H₅)₄ and LiCF₃ SO₃, and mixturesthereof.
 35. The method of claim 25 including selecting the solidcathode from the group consisting of silver vanadium oxide, coppersilver vanadium oxide, manganese dioxide, cobalt oxide, nickel oxide,fluorinated carbon, copper oxide, copper sulfide, iron sulfide, irondisulfide, titanium disulfide and copper vanadium oxide, and mixturesthereof.
 36. The method of claim 25 including providing the anodecomprised of lithium or a lithium-aluminum alloy.
 37. The method ofclaim 25 including providing the cathode comprising from about 80 toabout 99 weight percent of the cathode active material.
 38. The methodof claim 25 including providing the cathode further comprising a bindermaterial and a conductive additive.
 39. The method of claim 38 whereinthe binder material is a fluoro-resin powder.
 40. The method of claim 38including selecting the conductive additive from the group consisting ofcarbon, graphite powder and acetylene black and metallic powder selectedfrom the group consisting of titanium, aluminum, nickel and stainlesssteel, and mixtures thereof.
 41. The method of claim 25 includingproviding the cathode comprising from about 0 to 3 weight percentcarbon, about 1 to 5 weight percent of a powder fluoro-resin and about94 weight percent of the cathode active material.
 42. The method ofclaim 25 including powering an implantable medical device with theelectrochemical cell.
 43. A method for reducing voltage delay in a pulsedischarging electrochemical cell activated with a nonaqueouselectrolyte, comprising the steps of:a) providing an anode comprising analkali metal; b) providing a cathode including a mixed metal oxidecomprised of vanadium oxide and a second metal "SM" selected from thegroup consisting of Groups IB, IIB, IIIB, IVB, VIB, VIIB and VIII of thePeriodic Table of the Elements, the mixed metal oxide having the generalformula SM_(x) V₂ O_(y) wherein 0.30≦x≦2.0 and 4.5≦y ≦6.0; c) activatingthe electrochemical cell with the nonaqueous electrolyte operativelyassociated with the anode and the cathode, the nonaqueous electrolytecomprising:i) a first solvent selected from the group consisting of anester, an ether and a dialkyl carbonate, and mixtures thereof; ii) asecond solvent selected from the group consisting of a cyclic carbonate,a cyclic ester and a cyclic amide, and mixtures thereof; iii) a nitriteadditive having the formula: (RO)N(═O), wherein R is an organic group ofeither a saturated hydrocarbon or heteroatom group containing 1 to 10carbon atoms or an unsaturated hydrocarbon or heteroatom groupcontaining 2 to 10 carbon atoms; and iv) an alkali metal salt dissolvedtherein, wherein the alkali metal of the salt is similar to the alkalimetal comprising the anode; and d) discharge the cell to deliver atleast one current pulse of an electrical current of a greater amplitudethan that of a prepulse current immediately prior to the pulse.
 44. Themethod of claim 43 including selecting the nitrite additive from thegroup consisting of methyl nitrite, ethyl nitrite, propyl nitrite,isopropyl nitrite, butyl nitrite, isobutyl nitrite, t-butyl nitrite,benzyl nitrite and phenyl nitrite, and mixtures thereof.
 45. The methodof claim 43 wherein the nitrite additive is present in the electrolytein a range of about 0.001M to about 0.20M.
 46. A method for reducingvoltage delay in a pulse discharging electrochemical cell activated witha nonaqueous electrolyte, comprising the steps of:a) providing an anodecomprising an alkali metal; b) providing a cathode including a mixedmetal oxide comprised of vanadium oxide and a mixture of copper and asecond metal "SM" selected from the group consisting of groups IB, IIB,IIIB, IVB, VIB, VIIB, and VIII of the Periodic Table of the Elements,the mixed metal oxide having the general formula Cu_(x) SM_(y) V₂ O_(z)wherein 0.01 ≦x≦1.0, 0.01≦y≦1.0 and 5.01≦z≦6.5; c) activating theelectrochemical cell with the nonaqueous electrolyte operativelyassociated with the anode and the cathode, the nonaqueous electrolytecomprising:i) a first solvent selected from the group consisting of anester, an ether and a dialkyl carbonate, and mixtures thereof; ii) asecond solvent selected from the group consisting of a cyclic carbonate,a cyclic ester and a cyclic amide, and mixtures thereof; iii) a nitriteadditive having the formula: (RO)N(═O), wherein R is an organic group ofeither a saturated hydrocarbon or heteroatom group containing 1 to 10carbon atoms or an unsaturated hydrocarbon or heteroatom groupcontaining 2 to 10 carbon atoms; and iv) an alkali metal salt dissolvedtherein, wherein the alkali metal of the salt is similar to the alkalimetal comprising the anode; and d) discharge the cell to deliver atleast one current pulse of an electrical current of a greater amplitudethan that of a prepulse current immediately prior to the pulse.
 47. Themethod of claim 46 including selecting the nitrite additive from thegroup consisting of methyl nitrite, ethyl nitrite, propyl nitrite,isopropyl nitrite, butyl nitrite, isobutyl nitrite, t-butyl nitrite,benzyl nitrite and phenyl nitrite, and mixtures thereof.
 48. The methodof claim 46 wherein the nitrite additive is present in the electrolytein a range of about 0.001M to about 0.20M.
 49. The method of claim 46wherein in the general formula x≦y.